A methanol sensor is an important component in a direct methanol fuel cell power system. The sensor is used to monitor the concentration of methanol solution fed to the fuel cell anode electrode. Based on the response of the sensor, active control of the methanol concentration to the desired level can be achieved.
Barton et al., xe2x80x9cA Methanol Sensor for Portable Direct Methanol Fuel Cell,xe2x80x9d145 J. Electrochem. Soc., pp. 3783-3788 (1998), report the design of such methanol sensor, based on studies (X. Ren et al., 1 Proton Conducting Membrane Fuel Cells, Ed. Gottesfeld et al., pg. 284 (The Electrochemical Society, Pennington, N.J. (1995)) of methanol transportation across a polymer electrolyte membrane such as that used in a direct methanol fuel cell. The two sensor electrodes were prepared similarly as fuel cell electrodes by applying catalyst, usually Pt containing catalyst powder, and recast ionomer mixture to the polymer electrolyte membrane to form membrane electrode assembly (MEA). The MEA is exposed to the methanol solution on one side, and the methanol flux across the membrane is electro-oxidized at the other side of the MEA by applying an external voltage across the two electrodes. With a sufficiently high electrode activity, achieved by both high catalyst layer activity and a high voltage across the two electrodes, a limiting current, which is approximately proportional to the methanol concentration for a low methanol concentration range, can be measured as the response of the sensor. H2 evolution reaction occurs concomitantly on the counter electrode that is exposed to the methanol solution.
For this type of sensor, the interference of oxygen from air has to be dealt with by either limiting the air access to the methanol electro-oxidation electrode or applying a high voltage (about 1 volt.) across the sensor electrodes to limit the oxygen electro-reduction reaction. Because the sensor is operated with an external power source to drive the electrode reactions, the sensor is characterized as operating in a driven mode, with the need of consuming external electric power.
The present invention provides a self-driven sensor, i.e., no applied voltage is required, to measure the concentration of methanol in an adjacent aqueous solution. The methanol sensor is generally a direct methanol fuel cell with a membrane electrode assembly (MEA) containing a methanol electro-oxidation anode and an air (O2) electro-reduction cathode. Methanol access from the aqueous solution to the anode side of the MEA is limited to diffusion through a physical barrier, which can be an aperture opening with defined geometry, a porous medium, or the like. The anode and the cathode are shorted together through an electrical connection so all the methanol that reaches the anode side is completely consumed at the anode with concomitant oxygen reduction at the cathode. All the reactions are self-driven and no external voltage source is required. The current passing through the shorting connection is limited by diffusion of methanol through the barrier so that the current is directly related to the methanol concentration in the aqueous solution.
Various advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention includes a sensor for outputting a signal related to a concentration of methanol in an aqueous solution adjacent the sensor. A membrane electrode assembly (MEA) is included with an anode side and a cathode side. An anode current collector supports the anode side of the MEA and has a flow channel therethrough for flowing a stream of the aqueous solution and forms a physical barrier to control access of the methanol to the anode side of the MEA. A cathode current collector supports the cathode side of the MEA and is configured for air access to the cathode side of the MEA. A current sensor is connected to measure the current in a short circuit across the sensor electrodes to provide an output signal functionally related to the concentration of methanol in the aqueous solution.